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At the end of the Permian, at the Paleozoic-Mesozoic boundary (251.0 ± 0.4 Ma ago), 96% of oceanic organisms became extinct. The extinction lasted three million years, but the most intense and abrupt event was 251.4 Ma ago. A series of more or less substantiated hypotheses was suggested to explain this catastrophe: anoxia, higher CO2 and H2S contents, fall of the sea level, volcanism, and impact events confirmed by several impact craters. The synchronous variation in many factors responsible for biodiversity reduction, including those without casual relations, proves the existence of a common cause of primarily cosmic origin.
ISSN 1028334X, Doklady Earth Sciences, 2011, Vol. 438, Part 2, pp. 750–753. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © M.S. Barash, 2011, published in Doklady Akademii Nauk, 2011, Vol. 438, No. 6, pp. 777–781.
At the end of the Permian, at the Paleozoic–Meso
zoic boundary (251.0
0.4 Ma ago), 96% of the oce
anic organisms became extinct. This was the greatest
mass biota extinction in the Earth’s history. The biodi
versity in the Late Permian included about 250 000 spe
cies, whereas after the disaster it decreased to less than
10 000. The bottom of the Permian–Triassic Panta
lassa Superocean, which occupied 2/3 of the Earth’s
surface, was nearly completely subducted. The frag
ments of the midoceanic rocks remained as accre
tionary blocks near the active continental margins.
The sediments of marginal seas are found in many
regions of the modern continents. The midoceanic
sediments deposited at the Permian–Triassic bound
ary include shallow carbonates of paleoatolls and
cherts as deep sediments. The study of the distribution
of paleontological objects shows that both the biota
extinction and global changes in environmental con
ditions occurred during two phases: (1) at the Middle–
Late Permian boundary (260 Ma ago) and (2) at the
Permian–Triassic boundary (about 251 Ma ago).
In the first phase, the amount of genera reduced at
the expense of the benthos (rugose corals, fusulinids,
brachiopods, bryozoans, echinoderms, etc.), which is
explained by anoxia. During the second phase, low
oxygen or anoxic conditions covered the entire water
column in the ocean (superanoxia). The biodiversity
was mainly reduced at the expense of nekton and
plankton (ostrakods, radiolarian, etc.). The global col
lapse of bioproductivity is considered to be the cause
of the mass extinction of radiolarians, the most impor
tant plankton species. The complete extinction of the
major benthic forms—fusulinids and rugose corals—
may have been governed by the extinction of symbiotic
algae which, in turn, was probably caused by insola
tion reduction. Ammonoids and nautiloids and
other predators (fishes and conodonts) were sub
jected a little.
The organism extinction at the Permian–Triassic
boundary was studied in detail in Meishan Province,
South China. Figure 1 demonstrates statistical analy
sis of the distribution of sea organisms. The extinction
lasted three million years, but the most intense and
abrupt event in an interval of less than 500 years
occurred 251.4 Ma ago. This coincides with a sharp
decrease in
and an increase of ~100 times in
the amount of volcanic microspherules. Pyrite inter
layers (evidence of anoxia) and volcanic ash are
observed at this boundary. The Ir content was found to
be an order higher than its background concentration
in the Upper Permian and Lower Triassic sediments
[12]. Finally, a fast transgression occurred that time
and a new cosmopolitan conodont species
H. parvus
Many hypotheses were suggested in order to
explain the extinction of organisms: disappearance of
ecological niches during the continental amalgam
ation into Pangea; hypersalinity; anoxia; higher CO
and H
S contents; fall of the sea level down to the min
imal value in the Phanerozoic; transgressions; volcan
ism; warming and acid rains as a result of volcanism
and methane release from gas hydrates; and a short
term fall in temperature. All these factors which
reduced the biodiversity are substantiated by paleon
tological, geological, geochemical, isotopic, and other
data. Some factors yield visible relations but other
relations are absent or unknown.
Causes of Mass Extinction of Sea Organisms
at the Paleozoic–Mesozoic Boundary
M. S. Barash
Presented by Academician A.P. Lisitsyn January 24, 2011
Received January 24, 2011
—At the end of the Permian, at the Paleozoic–Mesozoic boundary (251.0
0.4 Ma ago), 96% of
oceanic organisms became extinct. The extinction lasted three million years, but the most intense and abrupt
event was 251.4 Ma ago. A series of more or less substantiated hypotheses was suggested to explain this catas
trophe: anoxia, higher CO
and H
S contents, fall of the sea level, volcanism, and impact events confirmed
by several impact craters. The synchronous variation in many factors responsible for biodiversity reduction,
including those without casual relations, proves the existence of a common cause of primarily cosmic origin.
Shirshov Institute of Oceanology, Russian Academy
of Sciences, Nakhimovskii pr. 36, Moscow, 117997 Russia
Why were the processes active in such a restricted
period of the geological scale so different and harmful
for the biota? Did asteroid impacts play any significant
role as occurred 65 Ma ago, at the Mesozoic–Ceno
zoic boundary?
The above factors did occur at the end of the Per
mian over about eight million years. The variations in
the internal geospheres and the Earth’s surface and
biosphere took place simultaneously. The relation of
biodiversity with tectonics, geoid evolution, mantle
convection, and even the nucleus shift that caused
geopolar change is suggested. The casual relations
were accomplished through the fluctuation in sea
level, volcanism, methane release, intensification of
the ocean stratification, anoxia, etc. (Fig. 2).
The variations in the ratio of some isotopes reflect
the deep changes in ecology at the Permian–Triassic
boundary. The
ranged from +5–7 to –2–4
which proves a sharp variation in bioproductivity. The
decrease in the
value down to the Phanero
zoic minimum is explained by the change in weather
ing in the course of land aridization. The drastic
decrease in
is related to warming. The shift of
values is explained by the bacterial sulfatereduc
tion of
during pyrite burial and the formation of
framboidal pyrite under strong euxinic conditions [11].
In the Phanerozoic, the Mg/Ca ratio in seawater
varied from 1.0 to 5.2, which is caused by the mixing of
the midoceanic hydrothermal and river waters. The
Mg/Ca ratio of seawater fluctuates inversely to the
global speed of the oceanic crust growth [10]. The
period from the end of the Paleozoic to the beginning
of the Mesozoic is an interval of mostly aragonite sed
imentation, so the global rifting rate was minimal,
which corresponds to the integration of Pangea.
By the modern views [7], the variations in different
systems of the Earth began about 265 Ma ago when the
Illavara polarity change occurred after the 50 Ma sta
bility of the geomagnetic field that provoked the fre
quent changes in the geomagnetic field over a long
time. This event which was caused by the changes in
the Earth’s core and mantle condition was shown on
the Earth’s surface five million years later in the series
of events mentioned above.
At the end of the Permian, the sea level fell down to
the Phanerozoic minimum that is related to the inte
gration of Pangea, and, first of all, the shelf communi
ties suffered. The shortterm fall of temperature was
probably contemporary with this event.
The changes in the climatic system took place at
the end of the Permian and beginning of the Triassic
[9]. The Hercynian Orogeny gradually stopped, which
decreased the chemical weathering of silicates and,
consequently, the supply of biogenic components into
the biosphere, which decreased more with warming.
The pole–equator thermal gradient weakened,
С, ‰
Fig. 1.
Stratigraphic distribution of species in the section of the Meishan Province and variation in
, simplified after [8].
upwellings decreased, and productivity and carbonate
precipitation decreased. After the completion of the
submergence of subpolar water, the ocean was quickly
filled with warm, low oxygen water. The transgression
has bought the anoxic condition in the shelf areas,
which was a cause of the mass extinction proved for the
end of the Permian by facial, geochemical, and biom
arker studies.
The large scale midoceanic and intraplate basaltic
volcanism is the most popular hypothesis of the Late
Permian extinction. The temporal coincidence
between basaltic eruptions in South China and the first
phase of biota extinction 260 Ma ago and also between
the giant eruptions of the Siberian traps and the main
extinction phase was traced. Acid and intermediate
volcanism was widely developed along the western
boundary of the Pantalassa Superocean. In South
China alone, the volcanoclastic layer covers a territory
of more than 1 million km
. Eruptive volcanism gener
ated huge masses of gases and ash, occurred at the
areas of carbonate sediments, and brought about the
great release of CH
and CO
volumes enlarging the
effect of the Siberian traps.
The eruption brought on the “volcanic winter”
accompanied with a global fall in temperature because
of the aerosol screening in the atmosphere, gas release,
and acid rains harmful for plants. After the main basal
tic eruption, the following “volcanic summer” hin
dered the recovery of biodiversity and heightened the
ocean stratification. The gas hydrate decomposition
led to the release of a huge amounts of CO
and onset
of a strong and disastrous greenhouse effect. The fast
global warming led to weakening of the upwellings,
ocean stagnation, and decrease in productivity.
Although the reviewed factors negatively influenced
the biodiversity, they developed slowly and did not
cause rapid mass organism extinction on a global
The impacts of large asteroids or comets could also
be responsible for the sharp variations in ecological
conditions. Evidence of impact events were found at
the Permian–Triassic boundary in several sections.
However, the real craters were discovered only in
recent years, such as, for example, the Bedout astrob
leme (18.18° S, 119.25° E) covered with sediments
and located in northwestern Australia, 25 km from the
coast [3]. It was studied with seismography and
gravimetry and was drilled by two wells down to 3000 m
deep. The central part of the crater 180–200 km across
with Ar/Ar ages of 250.1 ± 4.5 and 253 ± 5 Ma is char
acterized by an uplift as in the Chixulub Crater.
Other craters formed at the Permian—Triassic
boundary are also known [4]. The largest in the South
America, the Araguainha Crater (16.77° S, 52.98° W,
40 km across), was found in Brazil. The fall of an aster
oid 2–3 km in diameter took place about 250 Ma ago.
The Arganaty Crater in Kazakhstan (49.5° N, 67° E,
315 km across, 250 Ma) is also a reliable crater. The
Falkland Crater (51° S, 60° W, 300 km across, 250 Ma)
is a probable astrobleme. The following craters are less
trustworthy: Great Kuonamki in the East Siberian
Platform (70° N, 111° E, diameter is not indicated,
251 Ma), Guli (70.91° N, 101.2° E, >50 km across,
251 Ma), Essey (68.81° N, 102.18° E, 4.5 km across,
Fig. 2.
Relation between abiotic factors and mass extinction of organisms at the Permian–Triassic boundary, modified after [13].
Sea level is shown after [6].
С, ‰
Integration of Pangea
System of Earth’s surface
20 2 4
Climate Space
phase 2
phase 1
mixed superchron
of internal geospheres
251 Ma), Alpine in Europe (43° N, 8° E, ~250 Ma),
and SAR 28 in Canada (56.57° N, 110.57° W, ~250 Ma,
7.5 km across).
Numerous fragments of chondritic meteorites with
typical geochemical features, impact quartz, metallic
grains, and cosmic fullerenes with incorporated
were found at the Permian–Triassic boundary layer in
the Antarctic, in the Graphitic Peak of the Central
Transantarctic Mountains [2]. Probably, Wilkes Land
in the Antarctic contains a trace of the impact of the
largest meteorite in the Earth’s history [5]. The large
negative magnetic anomaly coinciding with a ring
depression 243 km wide, 848 m minimal depth, and a
center at 70° S, 120° E was found here. The NASA
subsurface radar mapping revealed a 500km crater
located beneath the EastAntarctic ice shield. It is
considered to be the impact trace of a 55km asteroid
exceeding 4–5 times the Chixulub Asteroid, which led
to the mass extinction of organism 65 Ma ago. Based
on geophysical data, this took place about 250 Ma ago.
Most likely, precisely this event, along with the large
Bedout and other events, was the most important
cause of the drastic mass extinction of organisms at the
Permian–Triassic boundary. Geological samples
under the Antarctic ice have not yet been obtained and
the astrobleme is needed to be proved.
The impacts of large asteroids, especially occurring
within a narrow temporal interval, should exert the
destructive influence on sea and land organisms [1].
They should include a decrease in daylight and varia
tion in temperatures, acid rains, and fires. The global
distribution of the dust clouds composed of cosmic
particles and particles of the Earth’s crust ejected from
the crater would reduce photosynthesis and violate the
entire food chain. The effect could be intensified by
fires. If the asteroid fell in the ocean, then steam rejec
tion may have caused the greenhouse effect. An aster
oid impact on carbonate rocks with high CaCO
content would have increased the concentra
tion of C
and sulfuric aerosols in the atmosphere,
which would have brought acid precipitates and
increase in temperature by several degrees.
Because the relation of extinction at the Creta
ceous–Paleogene and Permian–Triassic boundaries
accompanied by the largest basaltic eruptions and
their synchronism with other harmful factors do not
cause doubts, it is suggested that the impact event
destroyed an already weakened biota. The simulta
neous variation in many factors determinative for
biodiversity, including those without any casual rela
tions, proves the existence of the common primary
cosmic cause.
The age estimation of the impact craters correlates
with the events of the mass organism extinction. In
order to explain the quasiperiodicity of both pro
cesses (about thirty million years), it is suggested to
examine the following cosmic events of similar fre
quency: crossing by the Solar system comet clouds of
the galactic arms, the vertical movements of the Sun
relative to the galactic plane, and the variation in the
gravitation potential of the Galaxy at various distances
from its center. As for the mass extinction of organ
isms, considering their suddenness and shorttime
interval, it may be explained only by the rapid cata
strophic changes in the environmental conditions
caused by asteroid impacts.
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(5649), 1388–1392 (2003).
3. L. Becker, R. J. Poreda, A. R. Basu, et al., Science
(5676), 1469–1476 (2004).
Catalogue of the Earth’s Impact Structures. Siberian Cen
ter for Global Catastrophes. Russian Academy of Sci
ences, Siberian Division,
5. R. R. von Frese, L. Potts, S. Wells, et al., Eos Trans.
, T41A08 (2006).
6. A. Hallam, Ann. Rev. Earth Planet. Sci.
, 205–243
7. Y. Isozaki, J. Asian Earh Sci.
(6), 459–480 (2009).
8. Y. G. Jin, Y. Wang, W. Wang, et al., Science
432–436 (2000).
9. D. L. Kidder and Th. R. Worsley, Palaeogeogr. Palaeo
climatol. Palaeoecol.
(3–4), 207–237 (2004).
10. J. B. Ries, Biogeosciences
(9), 2795–2849 (2010).
11. P. B. Wignall and R. J. Twitchett, Geol. Soc. Amer.
Spec. Pap.
, 395–413 (2002).
12. D.Y. Xu and Y. Zheng, Palaeogeogr. Palaeoclimatol.
(1–4), 171–176 (1993).
13. H. Yin, Q. Feng, X. Lai, et al., Glob. Planet. Change
(1–3), 1–20 (2007).
... As for biodiversity, geoengineering climate using aerosol injection may provide huge array of problems for life on Earth. Multiple studies indicated that the adverse effects of aerosol injection include intensification of ocean acidification, disruption of regional precipitation, possible enhancement of air pollution, increase frequency of acid rains, and possible contributions to adverse side-effects in human health [4,8,21,22,23]. Other studies focusing on evaluating biodiversity to historical volcanic eruption have also found that species tend to go extinct after major volcanic eruptions and subsequently replaced by other species that able to adapt and survive, thus completely changing the region ecosystem [21,24]. ...
... Multiple studies indicated that the adverse effects of aerosol injection include intensification of ocean acidification, disruption of regional precipitation, possible enhancement of air pollution, increase frequency of acid rains, and possible contributions to adverse side-effects in human health [4,8,21,22,23]. Other studies focusing on evaluating biodiversity to historical volcanic eruption have also found that species tend to go extinct after major volcanic eruptions and subsequently replaced by other species that able to adapt and survive, thus completely changing the region ecosystem [21,24]. ...
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Eustasy can be studied using a variety of methods, including areal plots of the changing temporal distribution of marine deposits, facies analysis of stratigraphic sequences, and seismic stratigraphy, allied with the best available means of biostratigraphic correlation. The results of these various methods are then compared for use in eliminating the complicating effects of local and regional tectonics in the interpretation of sea-level oscillations. The determination of the rate and amount of sea- level change is also discussed. Use is made of areal plots, in conjunction with hypsometric data and a variety of stratigraphic sequence evidence, to produce a eustatic curve for the pre-Quaternary Phanerozoic. Notwithstanding its necessarily tentative and provisional nature, this curve is considered to be a more accurate representation of Phanerozoic eustasy than that of Vail et al (1977).-from Author
Permian waning of the low-latitude Alleghenian/Variscan/Hercynian orogenesis led to a long collisional orogeny gap that cut down the availability of chemically weatherable fresh silicate rock resulting in a high-CO2 atmosphere and global warming. The correspondingly reduced delivery of nutrients to the biosphere caused further increases in CO2 and warming. Melting of polar ice curtailed sinking of O2- and nutrient-rich cold brines while pole-to-equator thermal gradients weakened. Wind shear and associated wind-driven upwelling lessened, further diminishing productivity and carbon burial. As the Earth warmed, dry climates expanded to mid-latitudes, causing latitudinal expansion of the Ferrel circulation cell at the expense of the polar cell. Increased coastal evaporation generated O2- and nutrient-deficient warm saline bottom water (WSBW) and delivered it to a weakly circulating deep ocean. Warm, deep currents delivered ever more heat to high latitudes until polar sinking of cold water was replaced by upwelling WSBW. With the loss of polar sinking, the ocean was rapidly filled with WSBW that became increasingly anoxic and finally euxinic by the end of the Permian. Rapid incursion of WSBW could have produced ∼20 m of thermal expansion of the oceans, generating the well-documented marine transgression that flooded embayments in dry, hot Pangaean mid-latitudes. The flooding further increased WSBW production and anoxia, and brought that anoxic water onto the shelves. Release of CO2 from the Siberian traps and methane from clathrates below the warming ocean bottom sharply enhanced the already strong greenhouse. Increasingly frequent and powerful cyclonic storms mined upwelling high-latitude heat and released it to the atmosphere. That heat, trapped by overlying clouds of its own making, suggests complete breakdown of the dry polar cell. Resulting rapid and intense polar warming caused or contributed to extinction of the remaining latest Permian coal forests that could not migrate any farther poleward because of light limitations. Loss of water stored by the forests led to aquifer drainage, adding another ∼5 m to the transgression. Non-peat-forming vegetation survived at the newly moist poles. Climate feedback from the coal-forest extinction further intensified warmth, contributing to delayed biotic recovery that generally did not begin until mid-Triassic, but appears to have resumed first at high latitudes late in the Early Triassic. Current quantitative models fail to generate high-latitude warmth and so do not produce the chain of events we outline in this paper. Future quantitative modeling addressing factors such as polar cloudiness, increased poleward heat transport by deep water and its upwelling by cyclonic storms, and sustainable mid-latitude sinking of warm brines to promote anoxia, warming, and thermal expansion of deep water may more closely simulate conditions indicated by geological and paleontological data.
The event across the Paleozoic–Mesozoic transition involved the greatest mass extinction in history together with other unique geologic phenomena of global context, such as the onset of Pangean rifting and the development of superanoxia. The detailed stratigraphic analyses on the Permo-Triassic sedimentary rocks documented a two-stepped nature both of the extinction and relevant global environmental changes at the Guadalupian–Lopingian (Middle and Upper Permian) boundary (G-LB, ca. 260 Ma) and at the Permo-Triassic boundary (P-TB, ca. 252 Ma), suggesting two independent triggers for the global catastrophe. Despite the entire loss of the Permian–Triassic ocean floors by successive subduction, some fragments of mid-oceanic rocks were accreted to and preserved along active continental margins. These provide particularly important dataset for deciphering the Permo-Triassic paleo-environments of the extensive superocean Panthalassa that occupied nearly two thirds of the Earth’s surface. The accreted deep-sea pelagic cherts recorded the double-phased remarkable faunal reorganization in radiolarians (major marine plankton in the Paleozoic) both across the G-LB and the P-TB, and the prolonged deep-sea anoxia (superanoxia) from the Late Permian to early Middle Triassic with a peak around the P-TB. In contrast, the accreted mid-oceanic paleo-atoll carbonates deposited on seamounts recorded clear double-phased changes of fusuline (representative Late Paleozoic shallow marine benthos) diversity and of negative shift of stable carbon isotope ratio at the G-LB and the P-TB, in addition to the Paleozoic minimum in 87Sr/86Sr isotope ratio in the Capitanian (Late Guadalupian) and the paleomagnetic Illawarra Reversal in the late Guadalupian. These bio-, chemo-, and magneto-stratigraphical signatures are concordant with those reported from the coeval shallow marine shelf sequences around Pangea. The mid-oceanic, deep- and shallow-water Permian records indicate that significant changes have appeared twice in the second half of the Permian in a global extent. It is emphasized here that everything geologically unusual started in the Late Guadalupian; i.e., (1) the first mass extinction, (2) onset of the superanoxia, (3) sea-level drop down to the Phanerozoic minimum, (4) onset of volatile fluctuation in carbon isotope ratio, 5) 87Sr/86Sr ratio of the Paleozoic minimum, (6) extensive felsic alkaline volcanism, and (7) Illawarra Reversal.The felsic alkaline volcanism and the concurrent formation of several large igneous provinces (LIPs) in the eastern Pangea suggest that the Permian biosphere was involved in severe volcanic hazards twice at the G-LB and the P-TB. This episodic magmatism was likely related to the activity of a mantle superplume that initially rifted Pangea. The supercontinent-dividing superplume branched into several secondary plumes in the mantle transition zone (410–660 km deep) beneath Pangea. These secondary plumes induced the decompressional melting of mantle peridotite and pre-existing Pangean crust to form several LIPs that likely caused a “plume winter” with global cooling by dust/aerosol screens in the stratosphere, gas poisoning, acid rain damage to surface vegetation etc. After the main eruption of plume-derived flood basalt, global warming (plume summer) took over cooling, delayed the recovery of biodiversity, and intensified the ocean stratification. It was repeated twice at the G-LB and P-TB.A unique geomagnetic episode called the Illawarra Reversal around the Wordian–Capitanian boundary (ca. 265 Ma) recorded the appearance of a large instability in the geomagnetic dipole in the Earth’s outer core. This rapid change was triggered likely by the episodic fall-down of a cold megalith (subducted oceanic slabs) from the upper mantle to the D″ layer above the 2900 km-deep core-mantle boundary, in tight association with the launching of a mantle superplume. The initial changes in the surface environment in the Capitanian, i.e., the Kamura cooling event and the first biodiversity decline, were probably led by the weakened geomagnetic intensity due to unstable dipole of geodynamo. Under the low geomagnetic intensity, the flux of galactic cosmic radiation increased to cause extensive cloud coverage over the planet. The resultant high albedo likely drove the Kamura cooling event that also triggered the unusually high productivity in the superocean and also the expansion of O2 minimum zone to start the superanoxia.The “plume winter” scenario is integrated here to explain the “triple-double” during the Paleozoic–Mesozoic transition interval, i.e., double-phased cause, process, and consequence of the greatest global catastrophe in the Phanerozoic, in terms of mantle superplume activity that involved the whole Earth from the core to the surface biosphere.
The Bedout High, located on the northwestern continental margin of Australia, has emerged as a prime candidate for an end-Permian impact structure. Seismic imaging, gravity data, and the identification of melt rocks and impact breccias from drill cores located on top of Bedout are consistent with the presence of a buried impact crater. The impact breccias contain nearly pure silica glass (SiO2), fractured and shock-melted plagioclases, and spherulitic glass. The distribution of glass and shocked minerals over hundreds of meters of core material implies that a melt sheet is present. Available gravity and seismic data suggest that the Bedout High represents the central uplift of a crater similar in size to Chicxulub. A plagioclase separate from the Lagrange-1 exploration well has an Ar/Ar age of 250.1 +/- 4.5 million years. The location, size, and age of the Bedout crater can account for reported occurrences of impact debris in Permian-Triassic boundary sediments worldwide.
Platinum metals are depleted in the earth's crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record. One prediction of this hypothesis has been verified: the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other. Four different independent estimates of the diameter of the asteroid give values that lie in the range 10 +/- 4 kilometers.